It's funny, 'cause if you ask someone who considers ISO part of exposure, and you ask them if f/2.8 1/100 ISO 100 and f/5.6 1/100 ISO 400 result in the same exposure for a given scene, it's exceedingly rare that the answer to this simple yes or no question will take the form of "yes" or "no".

GB, ask them this: if they consider these 2 exposures to be equivalent, what about the 2x worse shot noise SNR in the ISO400 shot due to the 1/4x smaller total number of photons collected during the exposure period?